R help - Dec 2018 - "subscript out of bounds" error when using koRpus+Tree Tagger

Hi,

I'm trying to do text corpus processing on some novels, with koRpus 
package and Tree Tagger. The script lists all txt files (11 in all) in a 
dir, and processes it one by one.

##########
rm(list=ls())
library(koRpus)
library(koRpus.lang.en)
set.kRp.env(TT.cmd = "/pathto/tree-tagger-english", lang =
"en")
outdir <- "/pathto/corpora"
corpdir <- paste0(outdir,"/","morrison11")

files <- list.files(path=corpdir, pattern = "*.txt", full.names =
F)
n <- length(files)

output <- file(paste0(outdir,"/calc_results_morrison11.txt"),
open="at")
for (i in 1:n) {
   cat(i," - ",files[i],"\n", file = output)
   tagged.results <- treetag(paste0(corpdir,'/',files[i]),
      treetagger="kRp.env")
   capture.output(flesch(tagged.results), file = output)
   cat("\n", file=output)
   capture.output(TTR(tagged.results), file = output)
   cat("\n", file=output)
   capture.output(textFeatures(tagged.results), file=output)
   cat("\n===========================\n", file = output)
}
close(output)
#########

The problem is, the script always throws the following error when it 
works on the last txt file and prematurely exits:

??Error in all.patterns[[word.length]] : subscript out of bounds

I can't figure out what this message means. the dir's are correct; 
there's no problem with Tree Tagger installation; n and files have the 
correct values.

Please help, many thanks!

Jiayue